A friend brought over a Heathkit SB 220 for repair. I thought
I'd tell you what I found.
I removed the chassis from the case to first find the ALC
electrolytic capacitor strewn about the bottom in confetti and
foil. One of the grid chokes appeared to have been hot.
I replaced the capacitor and the entire alc terminal strip
parts assembly, then spent some time slowly reforming
the HV capacitors back up. The disabled and retired
owner has little or no money for new capacitors.
So I bring the unit up in slowly on a home brew fully metered
special variac unit with a dmm on the alc supply to view the
proper operation of the new terminal strip parts. Near 80%
of normal line voltage, the ALC 120 volt supply went up to
above 350 VDC, very strange indeed. At that point I thought
about a transformer break down. After much work to isolate
hi-pot and load test the filament transformer, it was deemed
healthy and returned to the regular amplifier wire harness.
Parasitics and or a tube problem might be the next thing to
investigate.
The unit fired up without tubes showed no problems at the
alc point, then each tube was tried in individually at both
locations. Again, no abnormalities observed.
When both tube were installed, the excessive ALC voltage
would again appear after a few minutes of warmup. I also
noted a few hundred volt drop in the HV supply. There also
appears to a problem with the Ip metering as the meter stays
at zero all the time. I'll deal with the meter circuit in the
next round. I removed the grid chokes and was quite
surprised to find them both open. A temporary strap from the
grid pins to the chassis for testing appears to eliminate the
excessive alc voltage.
I'll modify and install the basic parts to return the amplifier
to stable operation before giving it back. The open grid
chokes caused the excessive alc voltage which pop'd the
alc filter cap. I'll be looking closely for oscillation to have
caused the grid chokes to open. The saga continues...
cheers
skipp
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